Language Instincts: The evolution of sexual signals

From October 28, 2006

This Thursday Al Uy from Syracuse University came to talk to the Ecology, Evolution and Behaviour group at Queen’s. Al spoke about his research on the ecology and evolution of visual signaling in birds.

Plumage colour in birds is known to be involved in conspecific communication, including the signaling of quality to potential mates. In many species, females can gain information about the condition or health of a certain male just by assessing the colour of his feathers. Sexual selection for honest signaling is often implicated in the evolution of brightly coloured plumage, much like the evolution of the peacock’s tail described in my last post.

Al Uy is interested in understanding how sexual selection might contribute to the speciation process, since changes in sexual signals can lead to reproductive isolation between two populations. He pointed out that this idea originated with Darwin, who noticed that often the only difference between closely related species is a sexually dimorphic trait such as male plumage colour. To understand why this might be, Al studies the plumage colour of small birds called the bearded manakins with an interesting mating system: the males gather at display sites called leks, where they clear an area of the forest floor and dance to attract females. There are several subspecies of bearded manakins, all with striking differences in male colouration (whereas the females are all plain and look alike). Here are two of the manakin subspecies Al works on (golden-collared and white-bearded males):

Golden-collared manakinWhite bearded manakin

The signal function of manakin beards

Al began his talk by discussing his investigation of the signal function of colour in the golden-collared manakin, which formed the groundwork for his research in signal diversification. Al and his students have found that the yellow colour of the male ‘beard’ plumage functions as a signal of male quality that females assess during male dancing displays, since males with brighter yellow beards tend to be larger, have higher display rates, and obtain more matings than their dull-bearded counterparts.

Al also tested whether the brightness of the male plumage translated into conspicuousness from the point of view of the female manakins. He used a model that took account of avian perception, ambient light during male display, the reflectance of male plumage and the reflectance of the visual background during display (the dancing court). Consistent with the predictions of sexual selection theory, male conspicuousness as calculated by this model was related to male mating success. In other words, more conspicuous males obtained more matings. Unexpectedly, Al found that the darkness of the visual background was actually more important in explaining the variation in mating success than plumage reflectance, suggesting an important role of the visual habitat in shaping conspicuousness and the evolution of lekking. Al thinks that the lek mating system might have arisen from males becoming more competitive for the specific areas of the forest providing the best visual background for display. In the near future Al plans to test this idea by comparing the reflectance properties of the lek habitat with other areas in the forest.

Why are there so many beard colours?

The next part of Al’s talk focused on his main research interest: understanding what factors might promote the diversification of sexual signals like manakin beard colour. The basic hypothesis he is testing is that changes in the visual habitat can drive the diversification of visual signals. In order for plumage colour to be conspicuous (and therefore most effective as a signal) it needs to match the available light, contrast the background, and be tuned to the receptivity of the target individuals (females). Changes in any of these factors could have promoted the evolution of the four different male colour types found in the bearded manakins.

Al plans to test this hypothesis in several ways. First, he will examine whether or not the evolution of retinal physiology might be driving the diversification of male colour. He plans test this idea by comparing the abundance of retinal cones between the different manakin subspecies. If females from different subspecies have retinas that are optimally sensitive to the beard colours of their mates, then changes in female perception might be driving changes in male beard colour.

Al is also testing his ideas about colour diversification by studying a hybrid zone of golden-collared and white-bearded manakin populations. This is an area of Costa Rica where a river divides populations of the two manakin subspecies, although the yellow plumage trait (found in golden-collared manakins) extends slightly into the white-beaded population. One hypothesis Al is currently testing is whether this trait introgression has occurred because yellow plumage is intrinsically more attractive to the manakin females, although at this point he can only speculate as to why this might be. Al is testing this possibility by examining mate choice in the area where both yellow and white males are competing for the same mates.

The other question Al would like to answer is: what stops yellow males from sweeping further into the white population? It seems unlikely that the river is a physical barrier isolating these populations since birds are certainly capable of crossing it. Instead, Al is looking into the possibility that a change in the visual habitat on either side of the river is maintaining the separation between these two populations. In other words, white birds in succeed in their habitat (despite some introgression of yellow males) because white is the most conspicuous and best colour for displaying in that habitat.

Although Al’s talk was a bit long, it was enjoyed by the audience at Queen’s for telling a complete story without being overly technical, and some went so far as to claim it was the “best talk ever” (Dev Aiama). You can read more about Al Uy’s research here.

Language Instincts: Sex and honesty, or the mating scene for peacocks

Choosing between two males

From October 28, 2006

It doesn’t take an expert to realize that the extravagant song, dance and plumage of male birds (see videos in my last post) are intended as some sort of communication to potential mates. However, the insight that sexual ornaments are signals was a critical one that provided a mechanism for sexual selection and vindicated one of Darwin’s most important ideas. Darwin, of course, was the first to point out that extravagant male traits persist despite their probable costs because females prefer them. The example Darwin often referred to is the enormous and flashy tail of the male peacock (you can see for yourself at this online collection of all of Darwin’s writings).

It wasn’t until the 1970s when Amotz Zahavi suggested his handicap principle that we really began to understand why females prefer certain males. Zahavi’s idea was that extravagant sexual ornaments should be thought of as signals, and these signals are not selected in spite of their costliness but because of their costliness. A costly signal is an honest one; only the highest quality males can express it to its fullest extent. Females should be selected to prefer traits that are costly for males to produce because preference for an honest signal should yield the most benefits. Preference for a dishonest signal (one that a low-quality male could fake) should be weeded out by natural selection.

Darwinians had to wait a long time for honest signaling theory, and they had to wait even longer for someone to test these ideas with respect to the peacock. In the late 1980s and early Marion Petrie began to study what peahens actually look for in a mate. Following the love lives of peafowl in a London zoo, Petrie found a correlation between the number of eye-spots on a male tail and the number of peahens willing to mate with him.

Petrie’s next step was to test this correlation experimentally by decreasing the number of eye-spots on the tails of some males (by simply cutting them off). The question was whether the number of eye-spots directly influences mating success. Her results? Males who had eyespots removed were significantly worse off in terms of their number of mates compared with their mating success in the previous year.

Petrie also looked at the quality of peacock offspring, and found that the larger the eye-spots of a peacock dad, the better his offspring survive, suggesting that males with more extravagant tails are high quality individuals. This means that peahens gain a genetic benefit for their offspring by mating with well-plumed males. So the tail of the peacock would appear to be an honest signal of male quality rather than an arbitrary preference after all.

One other intriguing result of the work done by Petrie and her colleagues is that peacocks tend to display close to their kin. This suggests that kin selection has shaped males to aid their kin in attracting more females, and may provide a reason why poor-quality males who don’t get any mates bother to display at all. Interestingly, experimental work has shown that peacocks are able to recognize their brothers even when they do not hatch from the same nest. My question is: is it possible that birds might use some sort of visual cues that involve plumage for kin recognition?

Language Instincts: Advertising sex

From October 24, 2006

…bird sex, that is.

Male birds will do some pretty bizarre things in the name of sex. Here are some examples of displays put on by males in order to advertise their quality to potential mates (all the clips are from David Attenborough’s “Life of Birds” series for the BBC).

The superb lyrebird of Australia builds a small mound in the forest and sings a complex tune combining his own songs with songs (and noises) that he picks up from his environment:

Superb Lyrebird

The birds of paradise of Papua New Guinea are a strange bunch. Here are some clips of the extraordinary visual displays performed by these males:

Birds of Paradise

Wilson’s bird of paradise also has a very strange pattern of ornamental plumage (this video is also hilarious because of Attenborough’s sneaky hiding places):

Wilson’s Bird of Paradise

More to come on the topic of sexual signaling soon!

Language Instincts: “Run, run as fast as you can, you can’t catch me…”


From October 5, 2006

What does the racquet-shaped tail of a turquoise-browed motmot (the bird seen at right) have in common with the tail of a deer and the rhyming gingerbread man of fairy tale fame?

They are all important signals in the communication with predators.

The turquoise-browed motmot has a strange looking tail. The two central tail feathers are elongated and designed with weaker barbs towards the ends of the feathers. These barbs wear away to give the feathers an unmistakable tennis-racket shape. When faced with a predator, the motmot will repeatedly wag its tail from side to side in an exaggerated, pendulum-like way (see video of a related motmot species performing the wag display here). Bold move, you might think – and you would be right. The wag display will often draw the one’s eye to a motmot that might not have been seen otherwise, and no doubt it has the same attention-grabbing effect on predators. So why do it?

A researcher from Cornell University, Troy G. Murphy, recently looked into this problem, studying motmot colonies that nest in abandoned quarries and wells in Mexico. He developed several hypotheses to explain the tail wagging behaviour, and then performed careful observations of the context of over 100 wag displays to discriminate between the explanations.

His hypotheses were as follows: (i) The motmot wags its tail as a warning alarm signal to other motmots, alerting them to the presence of the predator. This means that the signal would be beneficial to nearby individuals (such as kin or mates) even though it might be dangerous for the signaler to draw attention to himself. (ii) The motmot wags its tail as a self-preservation alarm signal. The signal should still be directed to other motmots, but instead of being dangerous for the signaler it might benefit him by encouraging other nearby motmots to move closer together or even mob (attack) the predator. (iii) The motmot wags its tail as a pursuit-deterrent signal directed at the predator itself. Much like the gingerbread man, the tail wag would say, “Run, run as fast as you can, you can’t catch me…” This kind of predator-prey communication would actually benefit both adversaries: the prey gets to stay where he is while the predator avoids wasting energy on what would probably be futile chase.

Murphy found that when presented with a predator, the turquoise-browed motmot will perform the wag display even if it is alone and not within sight of other motmots. He also found that motmots are just as likely to tail-wag when they are alone as when they are near their mate, or near any other motmots. These observations allow us to reject (i) since the display is obviously not intended for motmot receivers. Murphy was also able to reject (ii) since the birds do not move closer together or mob when a predator approaches.

From this evidence we can conclude that the tail wagging must be a signal to the predator, communicating that the motmot has spotted the threat and is ready to escape. Interestingly, the tail of many ungulates has a similar pursuit-deterrence function. For example, some white-tailed deer will signal to chasing predators by flagging their conspicuous tails. Pursuit-deterrence signals have also been observed in lizards (arm-waving to deter predators) and fish (swimming right up and inspecting predators directly to deter them). Too bad for the gingerbread man – if he had stayed in one place and relied on his signaling he might not have been eaten after all.